During the fabrication of microelectronic circuits, a semiconductor substrate is typically coated at least once with photoresist or other masking material. Generally, photoresist is applied to completely cover the surface of the substrate. The photoresist layer is then exposed to ultraviolet light, or other radiation, through a mask containing the circuit patterns to be formed. Depending on the type of photoresist employed, either the exposed or non-exposed portions of the photoresist are then removed. Conventional etching or deposition steps may then be performed on the substrate. Thus, the photoresist protects the underlying portions of the substrate during microelectronic circuit fabrication.
For large scale integration, it is important that a uniform and defect-free photoresist be deposited. The most common technique for applying photoresist to a substrate is to place a drop of photoresist on the center of the substrate and then rotate the substrate to spread the photoresist over the substrate. Other alternatives to this spin-on technique have also been developed. For example, an article by Hochberg, Hoekstra, and Pennington entitled Depositing Photoresist in Thin Layers Independent of Surface Area, IBM Technical Disclosure Bulletin, Vol. 19, No. 6, pp. 2239-2240, Nov., 1976 describes an apparatus for uniformly depositing a photoresist mist onto the surface of a substrate or other object at atmospheric pressure. In particular, the apparatus includes an ultrasonic nebulizer to form the mist, and the substrate itself is statically charged to a polarity opposite that of the mist particles to thereby attract the mist particles to the substrate surface.
U.S. Pat. No. 4,290,384 to Ausschnitt et al. discloses a method and apparatus for coating a substrate with a mist of photoresist formed in a first chamber and transported to a deposition chamber by a carrier gas. Once in the deposition chamber, the photoresist mist settles onto the substrate under the force of gravity and under the influence of a sonic transducer to increase the settling rate of the mist. U.S. Pat. No. 4,989,541 to Mikoshiba et al. also describes a vertically oriented apparatus wherein discharged particles impact the surface to be coated under the influence of gravity and a control gas flow. The control gas forms a sheath around the particles and prevents the particles from impacting the walls of the reactor chamber and from being influenced by any heat convection turbulence.
For most semiconductor processing, each of several photoresist applications is followed by a conventional semiconductor processing step, such as etching, deposition, sputtering or plasma oxidation. The semiconductor processing steps typically require that the substrate be placed in an evacuable chamber and that the chamber be pumped down to subatmospheric pressure. Unfortunately, conventional spin-on techniques and the other techniques described above for applying photoresist or other coatings require that the substrate be vented and brought back to atmospheric pressure after a vacuum processing step has been performed. Moreover, repeated vacuum processing steps require that the evacuable chamber be repeatedly vented and then pumped down for each subsequent processing operation.
When a chamber is repeatedly pumped down and vented, particles may be formed and re-entrained in the chamber to eventually deposit on the substrate. Such particle contamination during vacuum processing may create defects in submicrometer semiconductor devices. The repeated venting and evacuation steps also greatly slow and reduce the production efficiency of the fabrication process. In addition, good uniformity of the photoresist coating on the substrate is important because induced striations and other nonuniformities may result in unreliable photoresist development which lead to unreliable or failed circuit elements in the finished microcircuit.